Chemistry Reference
In-Depth Information
CHAPTER 8
Atomistic Simulation of Ionic Liquids
Edward J. Maginn
Department of Chemical and Biomolecular Engineering,
University of Notre Dame, Notre Dame, Indiana 46556
INTRODUCTION
What exactly is an ionic liquid? The most straightforward and general
definition is that an
ionic liquid
is a liquid composed completely of ions. By
this admittedly tautological definition, NaCl is an ionic liquid if it is above
800
C. A eutectic mixture of LiCl and KCl above 355
C would also be an
ionic liquid. However, these compounds are not what people mean when
they talk about ionic liquids these days. ''Normal'' high-melting salts are gen-
erally referred to as
molten salts
, while the term
ionic liquid
has come to refer
to salts with much lower melting points than NaCl or LiCl-KCl mixtures. A
rather arbitrary definition that has been generally agreed upon in the research
community is that an ionic liquid (IL) is a salt with a melting point below
100
C. Salts that melt below room temperature are referred to as
room tem-
perature ionic liquids
, or RTILs. Measuring a melting point of an ionic liquid
accurately is nontrivial, however, so the definition is used only as a guide.
Many ILs form glasses and do not show an abrupt melting transition and
can actually be induced to crystallize by heating the liquid. This
cold crystal-
lization
phenomenon is typical of materials having sluggish dynamics and
reflects the fact that supercooled liquids will crystallize if they are given
enough thermal energy to reach their thermodynamically favored crystalline
state from a supercooled metastable state.
Ionic liquids have gone from academic curiosity to a widely used
(if not quite yet common) class of material in the span of about 15 years. In
